Showing papers on "Enthalpy published in 1997"

Molecular dynamics study of water clusters, liquid, and liquid-vapor interface of water with many-body potentials

Abstract: The molecular dynamics computer simulation technique is used to develop a rigid, four-site polarizable model for water. The suggested model reasonably describes the important properties of water clusters, the thermodynamic and structuralproperties of the liquid and the liquid/vapor interface of water. The minimum energy configurations and the binding energies for these clusters are in reasonable agreement with accurate electronic structure calculations. The model predicts that the water trimer, tetramer, and pentamer have cyclic planar minimum energy structures. A prismlike structure is predicted to be lowest in energy for the water hexamer, and a cagelike structure is the second lowest in energy, with an energy of about 0.2 kcal/mol higher than the prismlike structure. The results are consistent with recent quantum Monte Carlo simulations as well as electronic structure calculations. The computed thermodynamic properties for the model, at room temperature, including the liquid density, the enthalpy of vaporization, as well as the diffusion coefficient, are in excellent agreement with experimental values. Structuralproperties of liquidwater, such as the radial distribution functions, neutron, and x-ray scattering intensities, were calculated and critically evaluated against the experimental measurements. In all cases, we found the agreement between the observed data and the computed properties to be quite reasonable. The computed density profile of the water’s liquid/vapor interface shows that the interface is not sharp at a microscopic level and has a thickness of 3.2 A at 298 K. These results are consistent with those reported in earlier work on the same systems. The calculated surface tension at room temperature is in reasonable agreement with the corresponding experimental data. As expected, the computed average dipole moments of water molecules near the interface are close to their gas phase values, while water molecules far from the interface have dipole moments corresponding to their bulk values.

Determination of the Acidic Properties of Zeolite by Theoretical Analysis of Temperature-Programmed Desorption of Ammonia Based on Adsorption Equilibrium

Abstract: A new method of theoretical analysis for temperature-programmed desorption (TPD) of ammonia to determine the acid amount and strength and its distribution from a one-time experiment is proposed on the basis of the equilibrium between gaseous and adsorbed ammonia, i.e., free readsorption of ammonia. The entropy change was assumed to consist of the constant phase-transformation term and the gaseous mixing term as a function of gaseous concentration of ammonia. The enthalpy change, namely adsorption heat, was assumed to have several kilojoules per mole of the distribution. Thus a simulated TPD curve could be fitted well with the experimental data observed on mordenite and ZSM-5 zeolites. From the parameter set that gave the best fitted curve, the acidic properties of zeolite were determined. The determined acid amount was close to the difference between the aluminum and sodium contents, [Al] − [Na], in most cases. This confirms a simple principle that one acid site is generated by isomorphous substitution of...

Thermodynamic study of the low-temperature phase B19′ and the martensitic transformation in near-equiatomic Ti-Ni shape memory alloys

Abstract: Experimental information on the transformation temperatures and the thermodynamic properties of the near-equiatomic TiNi alloys is analyzed. Special attention is paid to the estimation of T0 temperature from experimental Ms and Af temperatures. The properties of the TiNi low-temperature phase (B19′) are evaluated from selected experimental data by using a two-sublattice model. The Ti-Ni phase diagram including the B19′ phase is then calculated. It reveals that the equiatomic TiNi parent phase (B2) remains stable from high temperatures until 370 K, and then the B19′ phase becomes thermodynamically stable as a linear compound under 370 K. Thermodynamic quantities such as the T0 temperature and transformation enthalpy are also calculated and compared with experimental data. Further, the Ms temperature is predicted and compared with data from different sources.

Thermodynamic and Nuclear Magnetic Resonance Study of the Reactions of α- and β-Cyclodextrin with Acids, Aliphatic Amines, and Cyclic Alcohols

Abstract: Titration calorimetry was used to determine equilibrium constants and standard molar enthalpy, Gibbs energy, and entropy changes for the reactions of a series of acids, amines, and cyclic alcohols with α- and β-cyclodextrin. The results have been examined in terms of structural features in the ligands such as the number of alkyl groups, the charge number, the presence of a double bond, branching, and the presence of methyl and methoxy groups. The values of thermodynamic quantities, in particular the standard molar Gibbs energy, correlate well with the structural features in the ligands. These structural correlations can be used for the estimation of thermodynamic quantities for related reactions. Enthalpy−entropy compensation is evident when the individual classes of substances studied herein are considered, but does not hold when these various classes of ligands are considered collectively. The NMR results indicate that the mode of accommodation of the acids and amines in the α-cyclodextrin cavity is ver...

Interaction of cations with SH-modified silica gel: thermochemical study through calorimetric titration and direct extent of reaction determination

Abstract: The reaction of silica gel with 3-(trimethoxysilyl)propane-1-thiol, MPTS, produced a thiol-modified silica gel, Sil–SH. This material removes silver, mercury, copper, zinc and nickel ions from aqueous solutions. Enthalpy changes that occur in the interactions between Sil–SH and these cations were obtained by a calorimetric titration technique, in which thermal effects and the quantities of cations that interact at each titration point were determined simultaneously. From this procedure, values of the maximum capacity of interaction (Ns), enthalpies of interaction at different coverages of the surface [Δint(i)Hm ] and integral enthalpy of interaction for a monolayer (ΔmonHm) have been established. It is shown that this method permits charting the basic sites created upon modification. From Δint(i)Hm values, the sequence of the interaction strength between the basic sites of the silica-attached MPTS and the cations is Hg2+>Ag+>Cu2+>Ni2+>Zn2+ , in agreement with the concept of hard and soft acids and bases. The influence of some parameters concerning the thermodynamics of solvation of the indicated transition metals has been also taken into account in the investigation of these interaction processes.

Adam-Gibbs Formulation of Enthalpy Relaxation Near the Glass Transition.

Abstract: The entropically based nonlinear AdamGibbs equation is discussed in the context of phenomenologies for nonlinear enthalpy relaxation within the glass transition temperature range. In many materials for which adequate data are available, the nonlinear Adam-Gibbs parameters are physically reasonable and agree with those obtained from linear relaxation data and thermodynamic extrapolations. Observed correlations between the traditional ToolNarayanaswamy-Moynihan parameters are rationalized in terms of the Adam-Gibbs primary activation energy (Dm ) determining how close the kinetic glass transition temperature can get to the thermodynamic Kauzmann temperature. It is shown that increased nonlinearity in the glass transition temperature range is associated with greater fragility in the liquid/rubber state above Tg.

Thermodynamic assessment of the V-N system

Abstract: A consistent thermodynamic data set for the V - N system is obtained by a computer-aided least squares method applied to all of the experimental phase diagram and thermodynamic data available from the literature. The sublattice model V 1 (N, Va) a is used to model the phases: fcc (a = 1), bcc (a = 3), and hcp (a = 0.5). The liquid phase is described by the Redlich-Kister formula, and the gas phase is treated as an ideal gas. Special attention is paid to the modeling of the fcc phase with its exceptional bulk of experimental data. This phase is first analyzed by an ideal solution, then by a regular, and finally by a subregular interaction in the nitrogen sublattice. The other solution phases are analyzed with similar modeling procedures. This step-by-step analysis procedure permits insight into reliable estimations for the parameters at each of the higher level models. Comparisons between the calculated and measured phase diagram and thermodynamic quantities show that most of the experimental information is satisfactorily accounted for by the thermodynamic calculation. Inconsistent experimental information is identified and ruled out. The thermodynamic property of the fcc phase in the V-N system is compared with those in the Cr-N and Ti-N systems and related to the Neumann-Kopp's rule.

Temperature and Pressure Dependence of the Multichannel Rate Coefficients for the CH3 + OH System

Abstract: Laser flash photolysis measurements on the kinetics of the title system in a He bath gas are reported for the temperatures 290, 473, and 700 K and a pressure range of 7.6−678 Torr. CH3 and OH radicals were generated simultaneously by 193 nm photolysis of acetone containing traces of water, with CH3 in large excess. The course of reaction was followed by monitoring the concentration of OH using laser-induced fluorescence (LIF), while CH3 was monitored by absorption at 216.4 nm. The experimental data were analyzed in the first instance by a simple model based on the decay of OH and CH3, which was verified by more comprehensive numerical simulations. Further analysis of the data, using a combined master equation/inverse Laplace transform/RRKM procedure, yielded estimates of the association rate coefficient of CH3 and OH ( = (8.0 ± 0.3) × 10-11 (T/300 K-0.79±0.09 cm3 molecule-1 s-1) and for the enthalpy change for the CH3 + OH → 1CH2 + H2O channel (Δ = 1.6 ± 2.0 kJ mol-1). Pressure-dependent rate coefficients...

Effects of protein glycosylation on catalysis: changes in hydrogen tunneling and enthalpy of activation in the glucose oxidase reaction.

Abstract: Three glycoforms of glucose oxidase, which vary in their degree of glycosylation and resulting molecular weight, have been characterized with regard to catalytic properties. Focusing on 2-deoxyglucose to probe the chemical step, we have now measured the temperature dependence of competitive H/T and D/T kinetic isotope effects and the enthalpy of activation using [1-2H]-2-deoxyglucose. The D/T isotope effect on the Arrhenius preexponential factor (AD/AT) is 1.47 (±0.09), 1.30 (±0.10), and 0.89 (±0.04) for the 136, 155, and 205 kDa glycoforms, respectively. The value obtained for the 136 kDa glycoform is well above the range expected for semiclassical−classical (no tunneling) reactions (upper limit of 1.22). The abnormal AD/AT is rationalized by extensive tunneling. The enthalpies of activation are 8.1 (±0.4), 11.0 (±0.3), and 13.7 (±0.3) kcal/mol for the 136, 155, and 205 kDa glycoforms, respectively. Apparently, less glycosylation results in more tunneling and a lower enthalpy of activation. The crystal s...

Thermodynamics of Peroxynitrite and Its CO2 Adduct

Abstract: The equilibrium constant, K3, of aqueous homolysis of peroxynitrous acid into hydroxyl and nitrogen dioxide free radicals was estimated to be 5 x 10(-10) M. This value was derived from a thermodynamic cycle by use of the experimentally known delta fH degree(ONOO-,aq) = -10.8 kcal/mol and the enthalpy of ionic dissociation of ONOOH(aq), delta H degree 1 = 0 kcal/mol, as well as of the entropy of gaseous ONOOH, S degree(ONOOH,g) = 72 eu. Furthermore we assumed the entropy of hydration of ONOOH, delta S degree 2, to be -25 eu, a value closely bracketed by the hydration entropies of analogous substances. The rate constant of radical recombination of OH. with NO2. to yield ONOOH, k-3, was resimulated from experimental data and found to be ca. 5 x 10(9) M-1 s-1. Together with the estimated K3, this yields the homolysis rate constant k3 = 2.5 s-1. This value is close to 0.5 s-1, the rate constant of formation of a reactive intermediate during the isomerization of peroxynitrous acid to nitrate. Our thermodynamic estimate is therefore consistent with substantial amounts of OH. and NO2. free radicals being formed in this process. The thermodynamic implications for the carbon dioxide/peroxynitrite system are also discussed.

Effect of temperature on the thermodynamic properties, kinetic performance, and stability of polybutadiene-coated zirconia.

Abstract: This article describes the results of a study of the effect of temperature on the performance of a reversed-phase material prepared by coating polybutadiene (PBD) on porous zirconia. We examined the effect of temperature on retention, efficiency, and stability of this phase. The thermodynamic properties were evaluated via the separation of alkylbenzenes and a set of tricyclic antidepressant drugs at different temperatures, while the intrinsic kinetic performance of the PBD phase at elevated temperatures was examined by using alkylbenzenes as probe solutes. Moreover, the thermal stability was determined by measuring the drift in k‘ while continuously pumping a mobile phase at 100 °C. We found that enthalpy changes were between −2 and −4 kcal/mol and that changes in selectivity varied with the type of solute. High temperatures improved the column efficiency by 30%, mainly by accelerating the solute diffusion rate in the stationary phase. Finally, the PBD-coated zirconia phase was very stable at a temperatur...

Microcalorimetry study of interaction between ionic surfactants and hydrophobically modified polymers in aqueous solutions

Abstract: The interactions of hydrophobically modified poly(acrylamide) (HMPAM) and unmodified poly(acrylamide) (PAM) with sodium dodecyl sulfate (SDS) or tetradecyltrimethylammonium bromide (TTAB) have been studied by flow microcalorimetry at 302.7 K. The mixing enthalpy curves were determined by mixing SDS and TTAB solutions above their critical micelle concentration with HMPAM or PAM solutions of different concentrations. The mixing enthalpy curves of SDS and TTAB solutions with water were also determined. The critical aggregation concentrations (cac) and the thermodynamic parameters have been derived from the differential enthalpy curves. Pronounced endothermic peaks were detected for all the systems. The peak heights of the endothermic curves decrease with increasing hydrophobicity of the HMPAM samples. The HMPAM hydrophobicity has no obvious effect on the values of the cac. The interaction between TTAB and the polymers is much weaker than that between SDS and the polymers.

Effect of Oxygen Surface Groups on the Immersion Enthalpy of Activated Carbons in Liquids of Different Polarity

Abstract: A series of carbons, prepared by nitric acid oxidation of an activated carbon and subsequent heat treatment at increasing temperatures to selectively reduce the oxygen surface groups, have been used to determine the enthalpy of immersion in liquids with different polarity (benzene, methanol, and water). The areal enthalpy of immersion for the carbon with low oxygen content follows the order benzene > methanol ≫ water, similar to that found for nonporous carbons. On the other hand, the presence of oxygen surface groups which evolve as CO2 upon heat treatment does not affect much the enthalpy of immersion in methanol or water, the groups evolving as CO being mainly responsible for the evolution of enthalpy of immersion. For nonpolar molecules such as benzene the enthalpy of immersion of the carbons is independent of the chemical nature of the carbon surface.

An investigation of the influence of solute size and insertion conditions on solvation thermodynamics

Abstract: In this paper we examine the influence of solute size and insertion conditions on solvent structural changes and excess thermodynamic properties in the infinite dilution limit. A general integral equation approach which can be applied under arbitrary conditions is given and isothermal-isochoric and isothermal-isobaric insertions are discussed in detail. Scaling relationships valid in the large solute limit are determined for both structural and thermodynamic properties. This is done by considering macroscopic thermodynamic relationships and explicit evaluation of low solvent density expansions of pair correlation functions. The hypernetted-chain and reference hypernetted-chain closure approximations are used to obtain numerical results for the insertion of hard sphere solutes of varying diameter into hard sphere, dipolar hard sphere and water-like solvents. The results obtained give a good deal of insight into the nature of solvation of inert solutes. It is shown that for all three solvents the excess properties are very well represented by a function obtained by summing terms proportional to the solute volume, surface area and diameter. One would expect such a result for large solutes, but here we show that this expression extrapolates all the way down to solutes comparable in size to the solvent particles. Further, it is shown that both the numerical value, and, more importantly, the physical interpretation of the excess thermodynamic properties strongly depend on the insertion conditions. Under all insertion conditions the chemical potential is a local property in the sense that it is completely determined by solute-solvent correlations which are important only in the immediate vicinity of the solute. However, this is not true of the excess energy, enthalpy and entropy which all contain nonlocal contributions arising essentially from changes in the actual or effective solvent density depending on the insertion conditions. We demonstrate that the nonlocal contributions can be very significant and therefore the excess energies, enthalpies and entropies often cannot provide useful information about solvent structure near solutes. This has significant implications for models which attempt to rationalize excess thermodynamics in terms of local solvent structure in the vicinity of solute particles.

Theoretical Study of OH and H2O Addition to SO2

Abstract: The G2 computational method is applied to the study of the hydroxyl radical oxidation of SO2 to SO3 as well as the hydrolysis of SO2 to H2SO3. A key intermediate in the oxidation process is the HOSO2 radical, which is predicted to have a S−OH bond enthalpy (ΔH298) of 26.2 kcal/mol, 4.3 kcal/mol lower than the currently accepted value of 30.5 kcal/mol. The radical is characterized by a 2c-2e S−OH bond with an unpaired electron delocalized into the π* orbital of the SO2 moiety. The hydrolysis of SO2 to H2SO3 was computed with and without a catalytic water. The SO2·H2O and SO2·2H2O complexes and transition states are very similar to those computed for SO3 plus water. The uncatalyzed reaction has an activation barrier of 33.9 kcal/mol, which is reduced to 20.0 kcal/mol with one catalytic water. Since the reaction of SO2 with two waters is nearly thermoneutral (4.5 kcal/mol endothermic), the reaction may be more amenable to thermodynamic study compared to the SO3 + 2H2O reaction, which is much more exothermic.

Average Voltage, Energy Density, and Specific Energy of Lithium‐Ion Batteries Calculation Based on First Principles

Abstract: The theoretical average voltage, energy density (energy per volume), and specific energy (energy per mass) based on the active electrode material have been calculated from first principles for two types of rechargeable lithium-ion batteries. In the charged state the two batteries consist of LiC{sub 6} and Mo{sub 2} electrodes (M = Mo and Ni). The calculation was performed using the linearized augmented plane wave crystal code WIEN95 based on density functional theory (DFT). The structure was calculated by varying the unit cell volume of the experimentally known crystallographic data with respect to the total energy. The calculated results are compared with measured values. The temperature dependence of the average voltage, energy density, and specific energy was demonstrated to be of minor importance. In the case of the LiC{sub 6}/NiO{sub 2} battery this was done by calculating the vibrational energy contribution to the enthalpy change using the cluster approximation and the Amsterdam density functional (ADF) molecular code based on DFT. The agreement between theoretical and experimental values opens up the use of first principles quantum chemistry in battery technology.

Absolute rate constants and arrhenius parameters for the addition of the methyl radical to unsaturated compounds : the methyl affinities revisited

Abstract: Absolute rate constants and their Arrhenius parameters are reported for the addition of the methyl radical to 21 monosubstituted and 1,1-disubstituted alkenes and to 6 benzenes in 1,1,2-trifluoro-1,2,2-trichloroethane solution. They are used to convert relative reaction rates known as methyl affinities from the work of M. Szwarc and others for about 250 additional unsaturated compounds to the absolute scale. An analysis shows that the addition rates depend on the reaction enthalpy but also indicates a moderate nucleophilic polar effect for the liquid-phase reactions. It is pointed out that this polar effect may be smaller in the gas phase.

Thermodynamics of moisture sorption in alfalfa pellets

Abstract: Experimental data on sorption isotherms of alfalfa pellets were used to determine the thermodynamic functions (differential heat of sorption, spreading pressure, net integral enthalpy and entropy) of the pellets. The thermodynamic functions were then utilized in modeling the specific heats of pellets as a function of moisture content. The estimated values of the thermodynamic functions at different moisture contents confirmed the results of an earlier study on the influence of moisture sorption on the physical integrity of the pellets. The changes in the thermodynamic properties and physical integrity were prominent when the moisture of the pellets were between 0.10 and 0.12 (mass fraction basis). Using the differential heat of sorption, it was determined that alfalfa pellets have higher affinity for moisture uptake when compared to wheat and lentils. The specific heat of the solids of alfalfa pellets had a parabolic relationship with moisture whereas the specific heat of water fraction increased...

A General Equation for Correlating the Thermophysical Properties of n-Paraffins, n-Olefins, and Other Homologous Series. 3. Asymptotic Behavior Correlations for Thermal and Transport Properties

Abstract: In this final paper of the series, asymptotic behavior correlations (ABCs) are presented for thermal properties (ideal-gas enthalpy and free energy of formation, ideal-gas heat capacity, enthalpy of vaporization, and liquid heat capacity) and for transport properties (liquid viscosity, thermal conductivity, and surface tension). The theoretical basis for asymptotic behavior of these properties is discussed. The correlations were developed using literature data for n-paraffins and n-olefins (1-alkenes) and give accurate and consistent predictions. They are preferable to existing correlations in most instances. For specific heat, viscosity, thermal conductivity, and surface tension, comparisons are made with high molecular weight, linear polyethylenes and other long-chain molecules.

The enthalpy of formation of thin film titanium disilicide

Abstract: Solid state reactions between thin films of titanium and amorphous silicon (a-Si) were studied using differential scanning calorimetry. Multilayered diffusion couples were heated from just above room temperature to 900 K while monitoring the heat flow from the sample during the reaction. From analysis of our data we have calculated the enthalpy of formation to be ΔHf=−62±5 kJ/mol for the reaction, a-Si+Ti→C49−TiSi2 and have subsequently estimated the heat evolved during the reaction c-Si+Ti→C54−TiSi2 as ΔHf=−56±5 kJ/mol. Based upon this estimate, we find the enthalpy as determined from thin film samples for this system agrees with previous studies which made use of bulk sampling techniques.